Gas metal arc welding, commonly referred to as "GMAW" or "MIG" welding is an arc welding process in which the arc is shielded by a gas from the ambient atmosphere with metal transferred to the workpiece through the arc from a consumable wire electrode. The consumable wire electrode is continuously fed into the arc at a preselected speed corresponding to a given deposition rate for a given wire size. The mode of metal transfer is dependent upon the operating parameters such as welding current, voltage, wire size, wire speed, electrode extension and the protective gas shielding composition. The known modes of metal transfer include short circuit, globular transfer, axial spray transfer, pulse spray transfer and rotating arc spray transfer. The short circuit, globular and pulse spray modes of metal transfer are low deposition rate processes in which metal is deposited at rates below about 7 lbs/hr. The rotating arc axial spray mode of metal transfer is a very high deposition rate process which is unstable at metal deposition rates below about 20 lbs/hr using electrode wire size diameters of 0.035 inches or larger. Below this minimum rate of metal deposition the rotating arc becomes erratic and/or is unsustainable. The axial spray mode of metal transfer is used for depositing metal at rates above the low deposition rates provided by the short circuit, globular transfer or pulse spray modes of metal transfer and below the very high deposition rates obtained in the rotating arc axial spray mode. However, conventional practice of gas metal arc welding in the axial spray mode of metal transfer is limited to a rate of metal deposition between about 5-12 lbs/hr using a wire diameter size range of between 0.035 to 0.052 inches. Above this deposition range the welding operation becomes erratic and unstable until a rotating arc is established by raising the wire feed rate to the minimum deposition rate required for a stable rotating arc.
The non-rotating arc axial spray mode of metal transfer is considered to be the preferred choice of metal transfer for all position GMAW welding where reliability, stability, high quality and at least a relatively high deposition rate is required. In the axial spray mode of metal transfer fine droplets of molten metal are pinched off the end of the moving wire and fed axially through the arc column to the weld pool. The pinch effect is caused by electromagnetic forces on the molten tip of the wire. In the conventional practice of axial spray transfer, metal transfer remains stable only up to a deposition rate of about 12 lbs/hr. The rotating spray arc was developed to increase the operating deposition range for GMAW welding. In the rotating arc mode of metal transfer the arc is physically rotated by electromagnetic forces in a helical pattern about a longitudinal axis. As the arc rotates a controlled stream of metal droplets are transferred from the electrode tip to the weld pool over a relatively wide area. The pattern of penetration into the workpiece provided by the non-rotating arc axial spray transfer mode is much deeper than is obtained in the rotating arc mode and is easier to control.